Page 48 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 2
Discussion
Bonobos did not show an attentional bias towards human facial expressions of emotion. When repeating this experiment in human participants, an attentional bias towards emotional expressions was observed. The result for bonobos was unexpected for several reasons. First, great apes and humans show some continuity in facial expressions of emotions in both morphology and function, making it likely that modulation of attention by emotions stretches beyond conspecifics. Indeed, the facial musculature of chimpanzees and humans are remarkably similar (Burrows et al., 2006), paving the way for potential functional similarities in emotional expressions. The starkest examples of how expressions are shared is the human smile and the equivalent bared-teeth display in apes, as well as human laughter and the relaxed open mouth (‘play face’) in apes (Parr et al., 2007; Van Hooff, 1972). Similarly, the expression of anger in humans is suggested to be equivalent to the bulging-lip display in chimpanzees and the tense ‘lip press’ in bonobos (De Waal, 1988; Parr et al., 2007). Moreover, some facial configurations of emotions appear only in humans (fear), or have not yet been studied in detail in apes (surprise, sadness). Overall, given the continuity between (some) expressions, it may have been plausible for bonobos to show a bias towards emotional expressions of humans.
A second reason for why we believe the result is unexpected is that there is some evidence that apes view emotional expressions of humans similarly as those of conspecifics. A study in orangutans showed that when presented with isolated facial expressions of humans and orangutans, they generally looked longer to emotional expressions as compared to neutral expressions, regardless of whether the expressions were of humans or conspecifics (Pritsch et al., 2017). Moreover, one study showed that apes (N = 32) have some understanding of the directedness and valence of human emotional expressions and use these expressions to infer desires (Buttelmann et al., 2009). Furthermore, Kano and Tomonaga (2010) examined how chimpanzees and humans view isolated neutral and emotional faces, and found that both species show similar facial scanning patterns regardless of whether the stimuli were of conspecifics or non-conspecifics. Crucially, in chimpanzees, scanning patterns changed according to the emotional expressions that were being viewed, but patterns were relatively similar across faces of humans and chimpanzees. Moreover, there is experimental evidence that shows that great apes can understand humans’ emotional facial expressions to some extent, for instance to infer desires (Buttelmann et al., 2009). These findings suggest that great apes
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